Return none in after event

[da-h]

Bugfix: Returinng None is not possible with after-events

Description

This MR fixes #80 by specifying internally if the eventCall object given to the before/after-events get a return-value from the actual event.
Before this MR, returning None was not possible in an event, when also registering an after-event, as that value was used internally to determine whether the eventCall-object is constructed with or without a return value in the event call stack.

Things done in this MR

• Added a helper-class no_result_given that gets overwritten any time an actual event (in contrast to a hook) gets called.

Check all before creating this PR:

• unit tests adapted / created